There has been a growing interest in the development and manufacturing of microscale fluid systems for the acquisition of chemical and biochemical information, in both preparative and analytical capacities. Adaptation of technologies from the electronics industry, such as photolithography, wet chemical etching and the like, has helped to fuel this growing interest.
One of the first areas in which microscale fluid systems have been used for chemical or biochemical analysis was in the area of capillary electrophoresis (CE). CE systems generally employ fused silica capillaries, or more recently, etched channels in planar silica substrates, filled with an appropriate separation matrix or medium. A sample fluid that is to be analyzed is injected at one end of the capillary or channel. Application of a voltage across the capillary then permits the electrophoretic migration of the species within the sample. Differential electrophoretic mobilities of the constituent elements of a sample fluid, e.g., due to their differential net charge or size, permits their separation, identification and analysis. In order to optimize the separation aspect of the CE applications, researchers have sought to maximize the electrophoretic mobility of charged species relative to each other and relative to the flow of the fluid through the capillary resulting from, e.g., electroosmosis. See, e.g., U.S. Pat. No. 5,015,350, to Wiktorowicz, and U.S. Pat. No. 5,192,405 to Petersen et al.
In comparison to these CE applications, the technologies of the electronics industry have also been focused on the production of small scale fluidic systems for the transportation of small volumes of fluids over relatively small areas, to perform one or more preparative or analytical manipulations on that fluid. These non-CE fluidic systems differ from the CE systems in that their goal is not the electrophoretic separation of constituents of a sample or fluid, but is instead directed to the bulk transport of fluids and the materials contained in those fluids. Typically, these non-CE fluidic systems have relied upon mechanical fluid direction and transport systems, e.g., miniature pumps and valves, to affect material transport from one location to another. See, e.g., Published PCT Application No. 97/02357. Such mechanical systems, however, can be extremely difficult and expensive to produce, and still fail to provide accurate fluidic control over volumes that are substantially below the microliter range.
Electroosmotic (E/O) flow systems have been described which provide a substantial improvement over these mechanical systems, see, e.g., Published PCT Application No. WO 96/04547 to Ramsey et al. Typically, such systems function by applying a voltage across a fluid filled channel, the surface or walls of which have charged or ionizeable functional groups associated therewith, to produce electroosmotic flow of that fluid in the direction of the current. Despite the substantial improvements offered by these electroosmotic fluid direction systems, there remains ample room for improvement in the application of these technologies. The present invention meets these and other needs.